linux-kernel - Re: [regression] cpuset,mm: update tasks' mems

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Message-ID: <4B839103.2060901@cn.fujitsu.com>
Date:	Tue, 23 Feb 2010 16:25:39 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Lee Schermerhorn <lee.schermerhorn@...com>
Subject: Re: [regression] cpuset,mm: update tasks' mems_allowed in time (58568d2)

on 2010-2-23 6:00, David Rientjes wrote:
> On Mon, 22 Feb 2010, Nick Piggin wrote:
> 
>> If you have a concurrent reader without any synchronisation, then what
>> stops it from loading a word of the mask before stores to add the new
>> nodes and then loading another word of the mask after the stores to
>> remove the old nodes? (which can give an empty mask).
>>
> 
> Currently nothing, so we'll need a variant for configurations where the 
> size of nodemask_t is larger than we can atomically store.
> 

Sorry, Could you explain what you advised?
I think it is hard to fix this problem by adding a variant, because it is
hard to avoid loading a word of the mask before

	nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);

and then loading another word of the mask after

	tsk->mems_allowed = *newmems;

unless we use lock.

Maybe we need a rw-lock to protect task->mems_allowed.

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